Clinical studies are currently in progress to evaluate functional modifiers of P-glycoprotein (Pgp), an efflux pump associated with resistance to cancer chemotherapy. However, the effects of these modifiers on a more recently discovered efflux pump, the multidrug resistance associated protein (MRP), have not yet been fully characterized. MRP is expressed in most human tissues and is overexpressed in several tumor types. For these reasons, we have investigated the effects of three prototype Pgp modifiers, which act by different modes on the function of Pgp, on the function of MRP in two MRP-overexpressing cell lines: UMCC/VP lung and MCF-7/VP breast cancer cells. Clinically optimal plasma levels of verapamil, cremophor, and PSC833 have been shown to completely block the function of Pgp in Pgp-over expressing cells. However, in the two MRP-over expressing cell lines, these modifiers only partially blocked the function of MRP and combinations of these optimal concentrations acted antagonistically. Similar antagonistic effects were seen with combinations of suboptimal concentration levels of these blockers, while these combinations resulted in synergistic effects in Pgp overexpressing cells. For two biophysical parameters measured at the plasma membrane, membrane fluidity and membrane potential, the effects of these modifiers were essentially similar in Pgp and MRP expressing cells. We suggest that the 170 kD Pgp and the 190 kD MRP glycoproteins, imbedded in the plasma membranes, respond differently to simultaneous effects of the investigated prototype resistance modifiers. These results also suggest that the identification of the specific mechanism of drug resistance is important for the selection of chemotherapeutic strategies to block the efflux pump on the cancer cell.